1. Field of the Invention
The present invention is directed to scanning probe microscopes, and, more particularly, to a method for determining the shape of a probe tip and for reconstructing a dilated image using the acquired data. It additionally relates to a method and apparatus for monitoring the integrity of a carbon nanotube.
2. Description of Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a tip and low forces to characterize the surface of a sample down to atomic dimensions. Generally, SPMs include a probe having a tip that is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample and a corresponding map of the sample can be generated.
The atomic force microscope (AFM) is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and which typically has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as strain gauges, capacitance sensors, etc. The probe is scanned over a surface using a high resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. This is accomplished by moving either the sample or the probe assembly up and down relatively perpendicular to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Similarly, in another preferred mode of AFM operation, known as TappingMode™ (TappingMode™ is a trademark owned by the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus, or the associated technique, e.g., “atomic force microscopy.”
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
Notwithstanding the fact that scanning probe microscopes are high resolution instruments, the ultimate resolution of the data obtained by a scanning probe microscope is limited by the physical characteristics of the tip of the probe. More particularly, there are limitations as to how small or sharp the tip can be made. As a result, when imaging fine (e.g., Angstrom-scale) sample features, the tip shape is typically reflected in the acquired data. Stated another way, the acquired AFM images are combination of both the original surface topology and the shape of the probe that was used to acquire the image. The superpositioning of the shape of the probe on the surface topology image is known as image “dilation” and is considered a distortion of the original topography. Because the imaged surface data contains both the features of the sample surface and the probe shape, accuracy is clearly compromised. For many applications, this limitation is negligible. However, for other applications, the degree of accuracy required to resolve the features of the sample surface is significantly greater such that tip shape introduces appreciable error into the acquired data. For instance, in the semiconductor fabrication industry, imaging features such as lines, trenches, and vias with single nanometer accuracy is desired as such features are in the range of 100 nm, and continually getting smaller. With typical tip widths in the range of about 70 nm, this is becoming increasingly difficult. Clearly, in this instance, the tip shape is reflected in the data and must be removed to accurately reconstruct the sample surface.
Moreover, the aforementioned problems can be compounded by the fact that complex sample surface topologies require a commensurate increase in tip shape complexity to image the sample surface. For example, samples may include undercut regions where a particular scan position in the X-Y plane may have multiple vertical or “Z” positions. One such sample feature 10 is shown schematically in FIGS. 1A and 1B and has undercuts 12 and 14 on opposite sides of the feature 10. Two types of known tip shapes are schematically illustrated in FIGS. 1A and 1B in interaction with that sample feature. In FIG. 1A, a probe tip 20 of a traditional scanning probe microscope includes a parabolic, or other pointed shape that is relatively easy to characterize. Tip 20 includes a shaft 22 and a distal end 24 that, although sharp, is typically at least slightly rounded at its active surface 26. (Generally the “active” surface is the point on the probe that is mathematically determined to be that point on the probe that actively interacts with the sample surface, even if other points on the probe actually interact with at least certain parts of the sample surface.) During a scan, which may be one in which the probe operates in an oscillating mode, for instance, tip 20 interacts with the sample feature 10 to image characteristics of the surface of that feature. This interaction may include actual probe-to-sample contact or may stop short of contact with other, near-surface effects of probe/sample interaction being detected.
The actual interaction between the probe tip 20 and the sample 10 occurs at points along the tip profile 28 in FIG. 1A that vary with sample feature shape and with the X position of the tip 20 relative to the sample feature. Importantly, the closest point to the sample is sometimes spaced from the surface of sample feature 10 because the active region 26 of the tip 20 is physically incapable of engaging the sample. This effect results in part from the geometry of the tip 20. Specifically, because the tip 20 is tapered, the active region 26 cannot engage vertical or undercut surfaces on the sample feature 10 as the probe 20 oscillates vertically. The resultant image is shown by the resultant data points 28 and encompasses the hatched regions in FIG. 1A, which illustrate the actual profile generated by scanning the feature 10. The image profile includes distortion resulting both from image dilation due to tip shape and artifacts resulting from the inability of the tip 20 to interact with some surfaces of the sample feature 10.
So-called “re-entrant probes” have been developed partially to address this and similar needs. A re-entrant probe tip is characterized by being wider at its distal end than at the “shaft” or section immediately above the end. It is capable of bilateral scanning of vertical and even undercut surfaces.
One widely used re-entrant tip is the so-called “critical dimension” or CD tip, an example of which is shown schematically at 30 in FIGS. 1B and 2. CD tips 30 have protuberances 32 and 34 in the scan or “x” direction at the bottom end 36, imparting a “boot” shape to the tip 30 when viewed from the “y” direction. The protuberances 32 and 34 can interact with complex surface features including undercuts 12 and 14 and, accordingly, allow imaging of such complex topographies. CD tips having a diameter as small as 30 nm are possible. They can be used to scan surface features having a size of on the order of 10 nm or even less.
However, a CD tip 30, like a traditional tip 20, introduces dilation into the acquired image because it physically occupies a volume or “active region” as shown at 38 that interacts with the sample. The resulting data points are shown at 40 in FIG. 1B. During the course of scanning feature 10, the tip/sample contact point translates across the distal end of the tip (i.e., translating from points 32 to 34 for example) while the tip reference position of the AFM remains fixed at point 38. Consequently, the AFM image incorporates both the shape of the sample feature 10 and the shape of the tip 30. This dilation effect is shown by the acquired data points 40 and the resulting hatched regions in FIG. 1B.
Dilated images using CD or other tips can be “reconstructed” by removing the image distortion due to the tip shape from the dilated image and, accordingly, obtain an image of the actual surface feature. Dilated images can be reconstructed using any of the variety of known techniques such as “slope-matching” and “erosion” reconstruction techniques, both of which are commercially available, for example, in the X3D CD AFM sold by Veeco Instruments, Inc. They are also described, for instance, in “Tip Characterization Surface Reconstruction Of complex Structures With Critical Dimension Atomic Force Microscopy.” G. Dahlen et al., 2005.
All reconstruction techniques require actual knowledge of the tip shape or at least critical parameters of that shape. These parameters include profile data of the portion(s) of the tip that interact with the sample during the imaging process. In a CD tip, for example, the portions requiring knowledge include at least the protrusions 32 and 34. The shape of at least these “interacting portions” therefore must first be determined in order to reconstruct the sample surface data from the dilated image data.
Tip shape determination is not a simple task. This is particularly true in the case of a CD tip which, as discussed above, has a rather complex shape. For instance, since virtually any portion of the tip 30, including the bottom 36, protuberances 32 or 34, or sidewalls above the protuberances 32 or 34, may take part in the sample interaction process, it is essential that the shape of all interacting portions of the tip be known for an accurate reconstruction. Tip shape can be acquired from a variety of techniques, such as AFM model based characterizer images, blind tip reconstruction, TEM (Transmission Electron Microscopy), SEM (Scanning Electron Microscopy) methods, or any combination of these methods. However, each method has its own limitations that restrict the utility of its application.
Blind tip reconstruction is a methodology that is based on an assumption that protrusions in the AFM image represent the self-image of the tip, which is equivalent to the statement that sharp features on the sample surface act as the probe to image the AFM tip. This method has proven useful in estimating the outer envelope of the tip geometries when using appropriate characterizer samples. However, blind tip reconstruction cannot be easily used to image complex tip shapes. Moreover, to make the characterizer durable results in material selection that also results in rapid tip wear. At present, this method has not been applied to CD AFM probes.
TEM is an imaging technique in which a beam of electrons is transmitted through the tip, resulting in an enlarged image to appear on a fluorescent screen or layer of photographic film, or to be detected by a CCD camera. TEM is very time consuming and requires painstaking sample preparation. This renders it unsuitable for automated instruments or applications in which throughput is important. It also can damage the tip surface during e-beam exposure. It also cannot be used in situ because it requires that the tip be placed in the sample holder of a transmission electron microscope.
In SEM, an electron beam is focused into a beam with a very fine focal spot sized 1 nm to 5 nm, which is then directed and scattered over a region of the sample surface. Interactions in this region lead to the subsequent emission of electrons, which are then detected to produce an image. SEM has limited resolution and a propensity to damage the tip surfaces when using sufficient magnification for shape characterization. In addition, SEM, like TEM, cannot be used in situ.
The drawbacks of other known tip shape determination techniques lead to the development of a model based characterizer approach for tip shape determination in which the tip scans a “characterizer” of known shape. This scan can be expressed as the equation:P=IcθSc  Equation I                Where:                    P=the shape of the tip;            θ=the erosion operator; and            Sc=the shape of the known characterizer.                        
For CD AFM, the most precise method of tip characterization heretofore available was a “distributed” characterizer consisting of two structures which must be individually scanned to determine the entire tip shape. Referring to FIG. 3, the first structure is typically an Improved Vertical Parallel Surface (IVPS) 50. The typical IVPS has a calibrated width (typically about 100 nanometers) but an unknown height and, accordingly, can only serve to provide a width measurement for the CD tip 30. It is formed from a line feature with extremely smooth sidewalls 52 and 54 and a uniform width. The IVPS 50 may, for instance, be formed from a single silicon crystal. Similar NanoCD™ structures are available from VLSI and are NIST traceable. They are formed from a polysilicon line protruding from a silicon substrate 56.
The second structure 60, shown in cross-section in FIG. 3, is shaped so as to allow re-entry into the feature in a reverse scan and to allow imaging of the CD tip 30 sidewalls, hence permitting the acquisition of tip profile data that is height-dependent. A Silicon Overhang Characterizer Structure or “SOCS” is suitable for this purpose. A SOCS is fabricated by a combination of vertical dry etching and anisotropic wet etching, thus producing a diamond-shaped trench in a silicon substrate 62. The upper surfaces 64 and 66 of the trench, which are the ones imaged during the characterization process, are upwards sloped and sharpened at their edges 68 and 70 to a radius of below about 10 nm to as little as about 1 nm in a subsequent oxidation step. Engagement of the large trench undercut, including the sharp edge 68 or 70, in a CD scan provides tip profile information and allows characterization of tips 30 with large flare. (“Flare” refers to the widening of the shaft 31 of tip 30 as one proceeds upwardly from its distal end.) The formation of a SOCS and the use of a SOCS to characterize tip shape are disclosed, for example, in U.S. Pat. No. 7,096,711, which is assigned to Veeco Instruments Inc., and the subject matter of which is hereby incorporated by reference in its entirety. The removal of the SOCS characterizer dilation from the image of the tip/SOCS scan is described in U.S. Pat. No. 6,810,354, which is also assigned to Veeco Instruments Inc.
While the two-stage characterization process described above is effective, it exhibits significant drawbacks and disadvantages.
For instance, the IVPS 50 and the SOCS 60 cannot be formed on a single structure. Hence, two separate structures must be scanned each time the tip 30 is characterized. This can be a very time-consuming process.
The IVPS 50 and SOCS 60 also have limitations that restrict throughput and/or repeatability. For instance, it has been discovered that IVPS width shows considerable lot-to-lot variation, as well as an average that is about 20% more than the fabricator-stated width. These deviations adversely affect both accuracy and repeatability of width measurement. Consequently, the IVPS 50 must be calibrated with a golden line or other reference structure such as a NanoCD prior to its use. This adds complexity and cost to the tip characterization process.
In addition, the properties of an IVPS 50 vary significantly along the length of a given IVPS. It is therefore important to carefully register the tip 30 to the same location along the length of the IVPS 50 during every characterization process using, for example, a so-called 2DSPM zoom technique. In this process, a two-line reference scan is first performed, and a y-reference position is defined using pattern recognition. Then, a second two-line reference scan is performed, and an x-reference position is determined. Finally, a high-resolution data scan is performed with relative offsets to the x and y. This need for registration considerably increases the complexity and time of the characterization process.
Moreover, the edge of a SOCS must be very sharp to provide precise tip profile data. The actual radius is unknown and must be assumed for removing (i.e., “eroding”) the characterizer edge radius on the acquired scan and thus allow high fidelity tip reconstruction. Typically, the radius estimates range from 1-5 nm. Any errors in this assumption result in a corresponding error in the reconstruction calculations. The characterizer edge uncertainty problem is compounded by edge non-uniformity and the fact that edge wear and breakage occur over time. For instance, an edge that initially has a radius of less than 5 nm may wear or break after repeated characterization interactions with a tip and, accordingly, have a radius of 10 nm or even larger. This change in edge radius is directly dilates the tip/characterizer image, yet only a constant edge radius value is removed during subsequent image reconstruction. As a result, the larger edge radius of a broken or worn characterizer edge translates into a larger reconstructed (or “deconvolved”) tip edge radius. This imparts a higher than actual tip vertical edge height during qualification analysis and can lead to an erroneous determination that the tip has failed. For instance, a worn SOCS vertical edge can lead to a determination that the vertical edge height of a CD tip has increased from 15 to 25 nm when, in fact, the tip vertical edge height is less than 20 nm. If the user-selected “failure threshold” is 20 nm, the operator (or automated software) will determine that the tip has failed and discard the tip when in fact it is perfectly usable. This is extremely undesirable given the fact that CD tips cost hundreds or even thousands of dollars a piece.
In addition, the angled shape of edge of a SOCS (having an included angle of about 54.7°) prevents it from making physical contact with the distal end of the probe 30. This inability leads to “blind zones” during characterization of re-entrant tips in which the “reentrancy” or overhang of the tip exceeds that of the characterizer. Consequently, the reconstructed tip shape retains residual distortion in this region. That distortion may then be transferred into the reconstructed sample image.
The need therefore has arisen to provide an SPM characterizing procedure having higher throughput than heretofore known procedures.
The need has additionally arisen to provide a tip characterization procedure that can be performed in a single scan, i.e., without disengaging the tip from the sample containing the first characterizer and moving to and engaging a second characterizer. Meeting this need will result in approximately halving the time for tip characterization and directly improves AFM throughput.
The need has additionally arisen to provide a tip characterizer that has precisely known dimensions which remain stable after repeated characterization cycles.
Finally, the need has additionally arisen to provide a tip characterizer that can make full physical contact with the CD AFM probe distal end.